by Dan Vergano, USA TODAY

by Dan Vergano, USA TODAY

The flattening over the past 15 years of a rise in the world's average surface temperature springs from a natural cooling pattern in the eastern Pacific Ocean, climate scientists reported Wednesday.

That leveling off fed part of the skepticism toward global warming predictions in recent years, but researchers behind the new report see this "hiatus" as a pause in an inevitable climb.

"Our results strongly confirm the role that (man-made) emissions are having on the climate," says climate scientist Shang-Ping Xie, senior author on the Nature journal study. "At one point over the long term, the effect we are seeing in the Pacific will stop. I'm confident the bigger increases in warming will resume."

For now, the "hiatus" in global warming has left average surface temperatures lodged about 1.4 degrees Fahrenheit above normal for the past century. The top 10 warmest years on record have all come since 1998 as a result, but none looks markedly warmer than another.

Climate scientists have disagreed over the cause of the pause in fairly regular increases in temperatures seen before 1998. Explanations range from the atmospheric cooling effects of recent volcanic eruptions to Asia's increased industrial smog or else natural cooling oscillations in ocean surface waters, as occurs between the well known warmer El Niño and cooler La Niña weather pattern in the Pacific Ocean.

"Our results show that Pacific cooling has indeed pulled down the average global surface increases seen from global warming," Xie says. Essentially, a persistent La Niña-like weather pattern across the tropical eastern Pacific, an area covering about 8.2% of the globe, has created a "cool spot," he says. That has balanced out temperature increases manifested elsewhere in continuing melting of the Arctic, rising sea levels and record summer heat waves across continents, he says.

Worldwide surface temperatures are calculated by averaging thousands of sea surface measures from ships and weather station readings across the globe. In the study, the team "forced" computer model versions of the global climate from 1970 to 2012 to include the recent persistent cooling recorded in the Eastern Pacific in its calculations. With that information built into the models, researchers matched the temperature and weather patterns that have happened in recent years, including last year's North American drought, better than the models that were run without them, the study reports.

"Really, this seems pretty straightforward. The climate is complicated, and natural variability can mask trends seen over century-long timescales," says climate scientist David Easterling of the National Oceanic and Atmospheric Administration's National Climatic Data Center in Asheville, N.C. He was not part of the study. Similar plateaus in temperature increases have occurred in seven to nine-year steps in the past, as Easterling and colleagues showed in a 2009 study. "A lot of people have the misconception that global warming means that temperatures should be steadily marching upward, but natural variability plays a big role on the decade scale," he says.

MIT's Susan Solomon is more skeptical of the Pacific Ocean cooling as an explanation for the flattening, saying "a chicken vs. egg problem" dogs the finding. "Did the sea surface temperatures cool on their own, or were they forced to do so by, for example, changes in volcanic or pollution aerosols, or something else? This paper can't answer that question."

Xie acknowledges the criticism, saying some evidence suggests global warming has transferred a portion of that heat to the ocean depths, which may explain the cooling of Pacific waters at the surface.

The hiatus has figured in public debate over climate "sensitivity," the effect that doubling greenhouse gas levels above those from the pre-industrial erawill have on the climate. Greenhouse gases such as carbon dioxide released by burning coal, oil and natural gas have a warming effect on the atmosphere, driving much of the increase in global warming seen over the past century, according to the U.S. National Academy of Sciences and numerous other expert bodies. The planet is nearly halfway to this doubling of greenhouse gases over pre-industrial levels. This is likely to trigger an eventual increase of about 5.4 degrees Fahrenheit in global average surface temperatures.

A March report in The Economist magazine questioned whether the hiatus points to an overestimate in climate predictions. "I think we have strengthened the case that what we are seeing is a cooling of the Pacific that will naturally end at some point, and the warming will resume," Xie says.